In recent years, the ever-growing quantities of electronic information stemming from the spread of the Internet and increasing quality of images in particular have entailed the ever-larger capacities of optical disks, which is one of the most principal information recording medium. The larger recording capacities have been implemented in the past in the form of CDs, DVDs, BDs, etc., with the focused spot size reduced by increasing the numerical aperture of the Object lens and shortening the wavelength of the light beam and with the number of recording layers increased. However, to implement ever-higher recording densities requires resorting to new storage techniques that are different from the existing techniques of increasing the numerical aperture and shortening the wavelength.
One of the promising next-generation storage techniques is the holographic memory. In general, information is recorded to the holographic memory in the following manners: a signal light beam carrying page data information modulated two-dimensionally by a spatial light modulator (SLM) is allowed to interfere with a reference light beam to generate an interference pattern that is fixed onto a disk-type recording medium (optical disk) as a refractive index distribution. When the incident angle of the reference light beam relative to the recording medium is changed, multiple types of information can be recorded simultaneously to a single recording location. The information thus recorded is read out as follows: emitting the reference light beam used in recording to the recording medium at the same angle as upon recording causes diffracted light to emanate from the interference pattern fixed on the recording medium, the diffracted light then being detected by a photodetector to reproduce the recorded page data information. In this manner, the holographic memory allows multiple types of two-dimensional information to be written to and read from a single recording location therein, so that high-density information can be recorded and readout at a high speed.
However, if the recording medium is tilted during reading, the amount of diffracted light from the interference pattern fixed on the medium is reduced, which makes it difficult to obtain a good readout signal. Also, while the flat recording medium is being driven so as to rotate, a displacement of the recording medium due to its own bend or vibrations can distort the beam of diffracted light detected by the photodetector from the medium during reading for example, leading to a degenerated readout signal.
As a countermeasure to the above problem, there is a known technique for using a motor and gear to tilt the spindle motor rotating the recording medium in a manner compensating for the tilt of the medium (e.g., see Patent Document 1).
There is also a known technique for supporting the recording medium edge with a ball or roller arrangement to compensate for the tilt of the recording medium (e.g., see Patent Document 2).
However, the problems with the structure of Patent Document 1 are that because the tilt of the recording medium rotating at a high speed is measured, and a relatively massive spindle motor to which the recording medium is fixed is inclined in keeping with the amount of the measured tilt, response speed becomes lower and the structure involved turns out to be complicated.
Meanwhile, the problem with the structure of Patent Document 2 is that because the tilt of the recording medium is controlled with rollers pressed fixedly to the recording medium edge, the structure is simple but the recording medium surface can be damaged and worn from contact with the rotating rollers.
In order to solve the above problems, there is a known technique for having vibration generators positioned opposed to each other above and below the recording medium in the rotation axis direction in such a manner that the generators do not have contact with the medium, the vibration generators generating vibrations to apply pressure fluctuations between the recording medium and the vibration generators to thereby control the displacement of the recording medium in the rotation axis direction (e.g., see Patent Document 3). With this structure, no chafe occurs on the recording medium because the medium is controlled from being displaced in a non-contact manner by means of the air pressure coming from the vibration generators.